Examples of direct-current power supply devices include an insulated AC-DC converter which is structured by, for example, a diode bridge circuit that rectifies an alternating-current power supply and a DC-DC converter that steps down the direct-current voltage rectified in the diode bridge circuit to convert the voltage into a direct-current voltage with a desired potential. One known example of such an insulated AC-DC converter is a switching power supply device in which a switching element connected in series with a primary-side winding wire of a transformer for voltage conversion is turned on or off by a PWM (pulse width modulation) control method, a PFM (pulse frequency modulation) control method, or the like to control the current flowing in the primary-side winding wire, so that the voltage induced by a secondary-side winding wire is controlled indirectly.
Incidentally, in the insulated direct-current power supply device, a rated load current (or maximum load current) is defined and if the over-current state in which current flowing on the secondary side increases to be the rated load current or more occurs, the power supply device maybe damaged. In view of this, a control circuit on a primary side is often provided with an over-current detection function and an over-current protection function that turns off the switching element upon detecting the over-current.
Examples of methods for detecting the over-current state of the output in the switching control type insulated direct-current power supply device include a method of monitoring voltage obtained by current-voltage conversion (peak value of voltage with a triangular waveform) by a resistor for current detection that is provided in series with the switching element on the primary side (for example, see Patent Literature 1).